A MAGNETIC PHASE-DIAGRAM DESCRIBING THE PINNING BEHAVIOR OF THE CERAMIC TL2BA2CA2CU3OY HIGH-TC SUPERCONDUCTOR

A magnetic phase diagram of the vortex state of ceramic Tl2Ba2Ca2Cu3Oy was derived from detailed magnetization studies in fields up to 2 T in the temperature range between 4 K and 90 K. By an extrapolation method a "depinning line" B*(T) has been estimated where the critical current density J(c) becomes zero well below H(c2) indicating the transition to the flux flow regime. In the region with nonzero critical current densities we mainly found evidence for collective pinning behaviour. Only at lower fields up to 1.6 T single vortex pinning might occur at temperatures up to about 80 K. The temperature dependence of J(c) gives evidence for the existence of two kinds of pinning centers. A large number of pinning centers with low activation barriers, therefore being active mainly at lower temperatures, might be responsible for the enhancement of J(c) below about 35 K.